Introduction

The plankton in Long Island Sound (LIS) harness energy from light. There have been routine measurements made of the concentration of chlorophyll-A  in LIS by the CT Department of Energy and Environmental Protection (CT DEEP) and archived at LISICOS.uconn.edu. Several authors (see for example, Dam et al., 2010) have noted that there is a large degree of variation in the concentration of chl-A but that there was an anomalously low period in 1998-2000 that has been unexplained. The data from CT DEEP survey station E1 illustrate this behavior.	Map of the coast of Long Island Sound showing the location of CT DEEP station E1 by the circle and the closest grid point in the NCEP–DOE AMIP-II REANALYSIS model.
	The time series of the near surface chlorophyll-A measurements by CT DEEP at station E1. The green symbols show the seasonal average of the spring data and the red symbols show the summer data. The log mean of the spring (March-May) and summer (June-August) are shown by the red and green lines. The period 1998 to 2000 is bounded by vertical lines on the left of the figure to isolate the interval in which the means of chl-A in both seasons were almost a factor of ten lower than the longer term mean. A second low year occurred in 2010 and it is highlighted by the lines on the right.

O’Donnell et al. (2014) speculated that since large variations in cloudiness were common in southern New England that could impact the inter-annual variability of the primary production in LIS.

Summary

We have explored the possibility that the anomalously low concentration of chl-A observed in surveys of LIS in 1998-99 were associated increased cloud cover in the spring and summer and, consequently, reduced light levels. We used the NCEP reanalysis product from a global atmospheric model to examine the pattern of cloudiness since the NCEP team invested in an extensive data quality review and their products have the advantage of consistency with many other observations. Unexpectedly, we find that the cloud cover was unusually low during the years of anomalously low chl-A.

The green time series shows the spring (March-May), and green shows the summer (June-August), cloud cover fraction over LIS from the NCEP Reanalysis-II. The black lines outline the intervals when the chl-A was low.

Though it is often the case that the chl-A is a measure of plankton productivity, it and that low light might lead to low plankton biomass, it also possible that when light is high the amount of chl-A needed per cell is reduced.  The observations that the low cloud cover was associated with low chl-A is consistent with that interpretation. It is also possible that the reanalysis products are not sufficiently highly resolved to predict the cloud cover at a coastal area like Long Island Sound. Though available data is limited, it is likely that a trend of the magnitude predicted (see the green line in the figure) should be detectable. If the reanalysis is correct, then the ecological consequences of a changing light regime deserve additional attention.

References

Dam, H.G., J. O’Donnell and A.N.S. Siuda (2010).A Synthesis of Water Quality and Planktonic Resource Monitoring Data for Long Island Sound. Final Rept. Long Island Sound Study, LI-97127501 

Kanamitsu, M., W. Ebisuzaki, J. Woollen, S-K Yang, J.J. Hnilo, M. Fiorino, and G. L. Potter. (2002). NCEP-DOE AMIP-II Reanalysis (R-2): 1631-1643, Bulletin of the American Meteorological Society.

O'Donnell, J., R.E. Wilson, K. Lwiza, M. Whitney, W.F. Bohlen, D. Codiga, T. Fake, D. Fribance, M. Bowman, and J. Varekamp (2014). The Physical Oceanography of Long Island Sound. In Long Island Sound: Prospects for the Urban Sea. Latimer, J.S., Tedesco, M., Swanson, R.L., Yarish, C., Stacey, P., Garza, C. (Eds.), ISBN-13: 978-1461461258

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